1. Field of the Invention
The present invention relates to a semiconductor device, a method of manufacturing the same, and a lead frame used therefor and, more particularly, to a technique of connecting an inner lead to a semiconductor pellet without using any bonding wire.
2. Description of the Prior Art
A technique most popularly used to connect an inner lead to a semiconductor pellet is a wire connection method. As shown in FIG. 1, a semiconductor pellet 20 is mounted and fixed on an inner lead 52. The pellet electrode (not shown) of the semiconductor pellet 20 is connected to an inner lead 51 by a bonding wire 54, i.e., a metal thin wire, and the resultant structure is encapsulated with a resin material 13.
In this connection method, a large area must be ensured for the inner lead to which the bonding wire 54 is connected, i.e., the inner lead 51 such that wire bonding is enabled, and a size reduction is difficult to attain. Additionally, since a noble metal such as Au is often used for the bonding wire 54 because of restrictions on various necessary solid state properties, the material cost increases. Furthermore, the bonding wire 54 is a thin wire and therefore has a poor mechanical strength regardless of its material. In an exposed state, the bonding wire 54 easily deforms or ruptures. Hence, measures for protecting the bonding wire must be taken in the manufacturing process. In addition, when an overcurrent flows to a semiconductor device manufactured using this method, the wire tends to fuse and rupture because of resistance heating of the wire, so the semiconductor device cannot be used under such conditions. That is, a problem of surge resistance arises.
As described above, the wire connection method has a lot of restrictions and problems. To solve some of these problems, Japanese Unexamined Utility Model Publication No. 3-21854 discloses a technique shown in FIG. 2. Referring to FIG. 2, the terminal portion of an inner lead 51 is made to overlap that of an inner lead 52. A semiconductor pellet 20 is inserted between the inner lead 51 and the inner lead 52 and connected to the two inner leads, and the resultant structure is encapsulated with a resin material 13.
When the technique disclosed in Japanese Unexamined Utility Model Publication No. 3-21854 shown in FIG. 2 is used, the semiconductor device can be made compact, unlike the wire connection method. In addition, without any bonding wire, the process of encapsulating the resultant structure with a molten synthetic resin can be completed within a short period of time.
However, to make the inner lead 51 overlap the inner lead 52, at least two lead frames are needed to form the respective leads. This results in a larger material cost for the lead frame than that of the wire connection method. In addition, when the semiconductor pellet 20 is to be inserted, the inner leads 51 and 52 and the semiconductor pellet 20 interfere with each other. The semiconductor pellet 20 tends to break by chipping. Furthermore, the bonding interface of the inner lead 52 is not always parallel to the electrode of the semiconductor pellet 20. Therefore, a process of forming a projecting electrode on the semiconductor pellet or a projecting bonding surface on the inner lead 52, or a process of inserting a bonding medium 55 between the electrode of the semiconductor pellet 20 and the inner lead 52 is necessary. This complicates the manufacturing process and also makes automation difficult, resulting in an increase in manufacturing cost.
Another prior art as shown in FIG. 3 is disclosed in Japanese Unexamined Patent Publication No. 4-93055. Referring to FIG. 3, one major surface of a semiconductor pellet 20 is bonded to the terminal portion of an inner lead 51, thereby mounting the semiconductor pellet 20. Thereafter, the terminal portion of an inner lead 52 is connected to a pellet projecting electrode 56 provided on the other major surface of the semiconductor pellet 20, and the resultant structure is encapsulated with a resin material 13.
As in the prior art shown in FIG. 2, in the prior art shown in FIG. 3 as well, the semiconductor device can be made compact, unlike the wire bonding method. In addition, the surge resistance increases. In the prior art shown in FIG. 3, the semiconductor pellet hardly breaks. However, a process of forming the pellet projecting electrode 56 is necessary, and as in the prior art shown in FIG. 2, two lead frames are needed to overlay the leads, resulting in an increase in material cost for the lead frames. Therefore, the manufacturing cost of the semiconductor device increases as a whole.
As a technique of solving some of the above problems in the prior arts shown in FIGS. 2 and 3, a semiconductor device shown in FIG. 4 is disclosed in Japanese Unexamined Patent Publication No. 60-241241. Referring to FIG. 4, the distal end portions of inner leads 51 of a lead frame which is covered with a metal suitably used as solder are connected to a pellet projecting electrode 56 of a semiconductor pellet 20 through a flat lead 57, and the resultant structure is encapsulated with a resin material 13.
Since only one lead frame is used for the semiconductor device shown in FIG. 4, an increase in material cost for the lead frame can be prevented. However, a process of connecting the pellet projecting electrode 56 and the inner leads 51 of the lead frame through the flat lead 57 is necessary. This makes the manufacturing process more complex than that in the prior art shown in FIG. 2 or 3 and also makes automation difficult. In addition, since the flat lead 57 must be prepared, the number of members increases to result in an increase in material cost. Furthermore, a process of forming a pellet projecting electrode to obtain the projecting electrode 56 or forming a projecting bonding portion on the flat lead 57 is needed. Therefore, the manufacturing cost increases as a whole.
A semiconductor device shown in FIGS. 5A and 5B is disclosed in Japanese Unexamined Patent Publication No. 62-35549. FIG. 5A is a horizontal sectional view, and FIG. 5B is a longitudinal sectional view. This semiconductor device solves some of the problems of the semiconductor devices shown in FIGS. 1 to 4. One lead frame is used in this device. The distal end portion of an inner lead 52 is bent sideways. A semiconductor chip 20 is mounted between the distal end portion of the inner lead 52 and that of an inner lead 51, and the resultant structure is encapsulated with a resin material 13.
In the semiconductor device shown in FIGS. 5A and 5B, the number of members decreases because one lead frame is used. However, plating must be performed for both the upper and lower surfaces of the lead frame, resulting in an increase in plating cost, as compared to the above-described prior arts. In addition, since the distal end portion of the inner lead 52 is bent, a width A of the semiconductor device after encapsulation cannot be decreased, and a large mounting area is required. Furthermore, since a process of bending and fully inverting the distal end portion of the inner lead 52 sideways is necessary, the lead frame transfer speed in the manufacturing process of the semiconductor device must be reduced, resulting in a decrease in productivity.
A similar semiconductor device as shown in FIG. 6 is disclosed in Japanese Unexamined Patent Publication No. 48-38070 in which one lead frame is used, and the distal end portion of an inner lead is bent. Referring to FIG. 6, the lead frame is constituted by support frames 58 and 59 and inner leads 51 and 52. A semiconductor chip 20 is mounted on an island, i.e., a wide portion at the distal end of the inner lead 51. The distal end portions of the inner leads 52 are bent and fully inverted sideways and connected to solder bumps serving as pellet projecting electrodes 56 of the semiconductor pellet 20.
In the semiconductor device shown in FIG. 6, the number of members is small because only one lead frame is used as in the device shown in FIGS. 5A and 5B. However, the solder bumps 56 must be formed on the semiconductor pellet 20 in advance. In addition, plating must be performed for both the upper and lower surfaces of the lead frame. This increases the manufacturing cost as a whole. Furthermore, in mounting the semiconductor pellet 20 on the inner leads 51 and 52, the inner leads 52 are bent and connected to the solder bumps 56 of the semiconductor pellet 20 before the lower surface portion of the semiconductor pellet 20 is fixed on the wide island of the inner lead 51. Since this operation is performed before the semiconductor pellet 20 is fixed to the inner lead 51, misalignment tends to take place between the semiconductor pellet 20 and the distal end portions of the inner leads 52, resulting in a bonding position misalignment of the inner leads 52. As in the prior art shown in FIGS. 5A and 5B, the lead frame transfer speed must be reduced.
As another prior art of this type, a semiconductor device shown in FIGS. 7A and 7B is disclosed in Japanese Unexamined Patent Publication No. 2-33956. The inner leads shown in FIG. 7B extend inward from support frames 58 and 59, and portions near the centers of them along the longitudinal direction serve as inner leads 51 and 52, respectively. The support frames 58 and 59 are connected through tie bars 53. A semiconductor pellet 20 is mounted on a wide portion at the distal end of one inner lead, e.g., the inner lead 51. The inner lead 52 is lifted together with the support frame 59 relative to the support frame 58 and the inner lead 51 (displaced in the direction coming out of the page of FIG. 7B), and at the same time, shifted in a direction indicated by an arrow in FIG. 7B relative to the support frame 58 and the inner lead 51. With this operation, the distal end portion of the inner lead 52 overlaps (overhangs) the semiconductor pellet 20 and is connected to the surface pellet electrode of the semiconductor pellet 20, as shown in FIG. 7A.
This semiconductor device can be made compact, and the lead frame transfer speed during the manufacturing process of the semiconductor device can be increased. As a matter of fact, however, with the planar shape shown in FIG. 7B, it is difficult to horizontally move the upper half portion of the lead frame relative to the lower half portion in the direction indicated by the arrow in FIG. 7B to attain a predetermined displacement and make the semiconductor pellet overlap the distal end portion of the inner lead 52 with good reliability. In addition, to selectively connect the inner lead 52 to the surface electrode of the semiconductor pellet 20, a bump electrode must be formed, or a projecting bonding portion must be formed on the inner lead 52. Furthermore, plating must be performed for both the upper and lower surfaces of the lead frame. This increases the member of manufacturing steps and the plating cost.
As described above, the prior arts have unique problems, respectively.
More specifically, the semiconductor device shown in FIG. 1 can hardly be made compact. The material cost for the bonding wire increases. The bonding wire tends to deform or rupture. The surge resistance of the semiconductor device in use is decreased.
In the semiconductor device shown in FIG. 2, the material cost of the lead frame increases. In the assembly process, the semiconductor pellet is susceptible to chipping and scratching. A process for forming a projecting electrode on the semiconductor pellet or a projecting bonding interface on the inner lead, or a process for inserting a bonding medium is needed. This complicates the manufacturing process and makes automation difficult to accomplish.
In the semiconductor device shown in FIG. 3, to form a projecting pellet electrode, the number of manufacturing steps increases. In addition, the material cost for the lead frame increases.
In the semiconductor device shown in FIG. 4, since the flat lead is used, the numbers of manufacturing steps and members increase. To form a projecting pellet electrode or a projecting bonding portion on the flat lead, the number of manufacturing steps increases.
In the semiconductor device shown in FIGS. 5A and 5B, since plating must be performed for both the upper and lower surfaces of the lead frame, the plating cost increases. The width of the semiconductor device can hardly be reduced. The lead frame transfer speed is low.
In the semiconductor device shown in FIG. 6, to form the solder bump on the semiconductor pellet, the number of manufacturing steps increases. Since plating must be performed for both the upper and lower surfaces of the lead frame, the plating cost increases. The lead frame transfer speed is low. Misalignment tends to occur between the semiconductor pellet and the distal end portion of the inner lead.
In the semiconductor device shown in FIGS. 7A and 7B, it is difficult to make the semiconductor pellet overlap the distal end portion of the inner lead. A process for forming a bump electrode or a projecting bonding portion on the inner lead is necessary. Since plating must be performed for both the upper and lower surfaces of the lead frame, the plating cost increases.